Because of a trend to make higher-integrity, higher precision, and higher-performance semiconductor devices, device power consumption is increased. Hence, more careful consideration to the operating characteristics resulting from temperature change is required.
Generally, CMOS input buffer circuits include CMOS inverters. A TTL voltage level applied as input to a CMOS inverter would be converted to a corresponding signal having a CMOS voltage level at the inverter output. However, the logic threshold voltage of a CMOS inverter varies with changes in temperature. At a low temperature, high level input characteristics worsen because the logic threshold voltage level is raised. At a high temperature, low level input characteristics worsen because the logic threshold voltage level is lowered.
Variations in the input characteristics which result from surrounding temperature changes cause many problems, such as instability in circuit operation and reduction in circuit speed.
The logic threshold voltage of a CMOS inverter is a function of the ratio between the gain constants of its p-channel and of its n-channel MOS transistor and of the threshold voltage of the device.
As the temperature is increased, the device gain constant .beta. is decreased to .beta.oc 1/.sqroot.T.sup.3 ("oc" meaning proportional to), because the mobility of the channel carrier is decreased by the increase in temperature. However, because the mobility of both holes and of electrons are affected by temperature to a similar extent, the ratio between the gain constants (.beta. ratio =.beta. n / .beta. p) is independent of temperature.
On the other hand, the threshold voltages Vtn and Vtp of the device would both decrease by the temperature coefficient of 2mV/.degree. K, responsive to an increase in temperature. For example, if the temperature is increased by 50.degree. C., the logic threshold voltage is decreased by 0.4 V. Thus, the low-level input characteristics of a CMOS input buffer circuit worsen at a higher temperature range, and the high-level input characteristics worsen at a lower temperature range.